fix(marketplace): use ./skills/ prefix for skill paths

[igord7737]

Summary

Skill directories in this repo live under ./skills/<category>/<name>/, but .claude-plugin/marketplace.json references them as ./<category>/<name>/ (missing the skills/ prefix). Claude Code's plugin loader resolves these paths literally, so every skill in 6 of 7 plugins fails to load with Path not found.

Root cause

Example: devsecops-skills plugin declares
```json
"skills": ["./devsecops/secrets-gitleaks", ...]
```
but the directory at that path doesn't exist. Actual location: ./skills/devsecops/secrets-gitleaks/SKILL.md. Same drift applies to appsec, secsdlc, compliance, incident-response, threatmodel, and offsec plugins.

Reproduction on `main` @ 6e25a4b:

1. `claude plugin marketplace add AgentSecOps/SecOpsAgentKit`
2. `claude plugin install devsecops-skills@agent-sec-ops-kit-skills`
3. `claude doctor` → `6 plugin error(s) detected: ... Path not found: .../devsecops/secrets-gitleaks (skills)` × 6

Fix

Prefix all 31 skill paths with `./skills/` so they match the on-disk layout. No skill content changes — manifest paths only.

Test plan

• `claude plugin uninstall devsecops-skills@agent-sec-ops-kit-skills`
• Patched `marketplace.json` locally with this PR's diff
• `claude plugin install devsecops-skills@agent-sec-ops-kit-skills` — succeeds
• `ls ~/.claude/plugins/cache/agent-sec-ops-kit-skills/devsecops-skills/*/skills/devsecops/` — all 6 skill dirs present
• `claude doctor` — 0 plugin errors (was 6 before fix)
• (reviewer) Repeat verification for any of the other 5 plugins (`appsec-skills`, `secsdlc-skills`, etc.) — same fix pattern applied to all